A VCSEL is a laser device formed from an optically active semiconductor region that is sandwiched between a pair of highly reflective mirror stacks, which may be formed from layers of metallic material, dielectric material or epitaxially-grown semiconductor material. Typically, one of the mirror stacks is made less reflective than the other so that a portion of the coherent light that builds in a resonating cavity formed between the mirror stacks may be emitted preferentially from one side of the device. Typically, a VCSEL emits laser light from the top or bottom surface of the resonating cavity with a relatively small beam divergence. VCSELs may be arranged in singlets, one-dimensional or two-dimensional arrays, tested on wafer, and incorporated easily into an optical transceiver module that may be coupled to a fiber optic cable.
In general, a VCSEL may be characterized as a gain-guided VCSEL or an index-guided VCSEL. An implant VCSEL is the most common commercially available gain-guided VCSEL. An implant VCSEL includes one or more high resistance implant regions for current confinement and parasitic reduction. An oxide VCSEL, on the other hand, is the most common laterally index-guided VCSEL. An oxide VCSEL includes oxide layers (and possibly implant regions) for both current and optical confinement.
VCSELs and VCSEL arrays have been successfully developed for single-mode operation and multi-mode operation at a variety of different wavelengths (e.g., 650 nm, 850 nm, 980 nm, 1300 nm and 1550 nm).
In general, it is desirable for single-mode VCSELs to exhibit single-mode behavior within a range of specified operating conditions, while complying with other performance specifications. In general, it is desirable to increase the single-mode optical power produced by a VCSEL. In addition, it is desirable to reduce the series resistance of a VCSEL and the divergence of the output optical beam produced by the VCSEL. It also is desirable to reduce the susceptibility of a VCSEL to damage by electrostatic discharge (ESD). In some prior approaches, both high series resistance and low ESD thresholds have been linked to small current apertures.
A high injection current is needed to generate high output power from a VCSEL. A higher injection current, however, causes the output beam to diverge through a well-known beam-steering effect that results from the interaction between the lateral modes of the VCSEL and current-induced changes in the refractive index in the wave guiding structure of the VCSEL. At low injection current, a VCSEL typically operates in the fundamental (i.e., lowest order) lateral mode, and the refractive index in the wave guiding structure remains substantially constant. At higher injection current, however, the mode structure becomes unstable due to ohmic heating and spatial hole burning effects. These changes allow higher order lateral modes to propagate with higher divergence angles, resulting in less efficient coupling between the VCSEL and the associated optical fiber used to transfer light from the VCSEL. For implant VCSELs, the mixing between different lasing modes often is detected as a kink in a graph of optical power plotted as a function of the injection current (often called the “L-I” curve) for the VCSEL.
The size of the current aperture of a VCSEL has been increased to increase the output optical power of the device. A larger current aperture also reduces the series resistance of the VCSEL. Larger current apertures, however, increase the likelihood that higher order lasing modes will propagate and mix with the fundamental mode. Extended cavity structures and optical anti-guiding structures have been introduced into the wave guiding cavities of VCSELs to suppress higher order lasing modes. Oftentimes, the introduction of such structures increases the complexity and reduces the reliability of the VCSEL manufacturing process.